Various terpolymers of a monovinylic aromatic compound, an acrylic compound and maleic anhydride are known in the art and have been prepared according to various processes. For example, the Wingler et al U.S. Pat. No. 4,141,934 discloses polymers of styrene and/or .alpha.-methylstyrene, a monoolefinically unsaturated monomer containing nitrile groups such as acrylonitrile and at least one additional monoolefinicially unsaturated monomer such as maleic anhydride which are formed using bulk polymerization methods. The bulk polymerization methods employ one or more initiators and pressures of from 1 to 20 bars. The Dufour U.S. Pat. No. 4,298,716 discloses a tetrapolymer of .alpha.-methylstyrene, styrene, maleic anhydride and acrylonitrile which is prepared by mass/suspension batch processes wherein the .alpha.-methylstyrene acts as a chain transfer agent.
The Lee et al U.S. Pat. Nos. 4,223,096 and 4,262,096 disclose styrene-acrylonitrile-maleic anhydride terpolymers which are formed by bulk polymerization methods in the presence of a rubber such as polybutadiene. The Lee et al processes employ a catalytic initiator and a chain transfer agent and are conducted in the presence of one or more solvents. The Zimmerman et al U.S. Pat. No. 3,336,267 discloses a process for the solution polymerization of styrene, maleic anhydride and a methacrylic or acrylic acid ester using from 5 to 50 weight percent of one or more solvents. The Liebig et al U.S. Pat. No. 4,167,543 discloses styrene-acrylonitrile-maleic anhydride terpolymers which are formed using a polymerization initiator and which are suitable for use in thermoplastic molding compositions with an ABS type copolymer.
Various processes for preparing styrene-acrylonitrile-maleic anhydride terpolymers which are known in the art are disadvantageous in one or more respects including, for example, they are relatively expensive, they require increased process control and/or they result in inferior terpolymer products.